When monitoring a power system to detect system faults, protection equipment is typically configured to perform a number of diagnostic, or monitoring, routines. One such routine includes determining whether the fundamental current frequency components are within an acceptable range or envelope. More specifically, the protection equipment is configured to periodically determine the fundamental power system frequency component of power system current signals for each phase. Once the current signal fundamental frequency component is determined for each phase, each component is compared to a preselected desired envelope. If the determined fundamental component is within envelope, then the subject diagnostic test is passed. If, however, the fundamental component is not within the envelope, such condition may be indicative of a potential fault or other problem. If such a condition persists, the protection equipment may operate to open the circuit associated with the out-of-range component. Once the circuit is opened, a utility worker typically must locate and correct the root cause of the out-of-range current component, and then close the circuit-breaking mechanism in the protection equipment.
In order to identify accurately the location of a fault, for example, the fundamental current component is analyzed to determine the distance from the protection equipment to the fault. In performing such analysis, it is desirable to remove decaying offsets from the current signal fundamental power system frequency component to improve accuracy. Decaying offsets usually occur in line currents during power system transients and are caused by the response of inductive and resistive impedances in the line. The correction for such decaying offset must compensate for the time varying nature of the offset. Once the decaying offset has been removed, the current signal fundamental frequency component can be analyzed to facilitate locating the fault.
Mimic circuits and filters are known which mathematically model transmission line behavior according to the function IZ(t)=i(t)xc2x7R+v(t), where v(t)=L(di(t)/dt). This output signal is the sum of two terms: the first term proportional to the product of the mimic resistance and the line current, and the second term proportional to the product of the mimic inductance and the time derivative of the line current. The function above is generally used to remove decaying offsets. Generally, a discrete Fourier transform (DFT) is used to determine the fundamental power system frequency component and harmonics of each line current.
The mimic technique has limitations. For example, the differentiation of the input signal amplifies high frequency harmonics, particularly in industrial devices such as motor drives and cycloconverters. The amplification of these high frequency harmonics can result in false current spikes, which can cause the protective relay to trip at an incorrect time. This problem has become more troublesome as the speed of protection equipment has increased.
In a digital implementation, mimic filters typically rely on the differentiation of two input samples. In order to compensate for a false signal spike, the differential gains are reduced by increasing the sampling period. However, the increase in the sampling period tends to decrease the bandwidth of the filter.
Other techniques are known. For example, to reduce the number of computations required to perform offset correction, U.S. Pat. No. 5,798,932 discloses the separation of fault detection and determination of fault location. More specifically, this patent recognizes that the decaying offsets do not necessarily have to be removed from the current phasors in order to determine whether a fault exists, and that the decaying offsets need only be removed when using the phasors to locate the fault. By reversing the order of process steps executed so that phasor values are generated and then, when needed, removing decaying offsets from the generated phasor values, the computational burden can be significantly reduced in certain applications.
U.S. Pat. No. 5,796,630 discloses a protective relay system intended in part to remove the influence of harmonic components that may be present in a fault current. The system includes a digital filter for outputting first and second difference electric variable data indicative of first and second differences between at least two sample data of first and second electric variables. The system also includes an addition filter for outputting first and second additive electric variable data indicative of orthogonal vector data with respect to the first and second difference electric variable data. The system further includes a relay control unit for calculating controlled variables of a relay operation in the power system on the basis of the first and second difference electric variable data at a certain sampling time, so as to determine whether or not protection of the power system should be carried out.
U.S. Pat. No. 4,577,279 discloses a method and apparatus for providing offset compenstation. The effects of a transient exponential noise signal are removed by sampling a sinusoidal signal, averaging the transient over a time interval corresponding to a number of cycles of the sinusoidal signal, and subtracting the average from the sample at the midpoint of the time interval.
While these and other techniques for dealing with offsets are known, none adequately provides a fast, reliable, and precise technique for filtering noise from power system signals supplied to protective relays while avoiding false signal spikes.
In view of the above discussion, it would be desirable to provide a reliable, fast, and precise technique for filtering power system signals. It would further be desirable for such a technique to avoid false signal spikes. It would still further be desirable for such a technique to be tunable for different applications, and for such a technique to be realizable in analog or digital circuits.
The present invention addresses the above concerns, and achieves additional advantages, by providing for a filtering technique which, according to exemplary embodiments, replaces the differentiation of a conventional mimic filter with a pseudo-differentiation to suppress low frequency signals. According to one example, the pseudo-differentiator consists of a forward gain Kp and an integrator in a feedback loop. Thus, according to an exemplary method of the present invention, the input electrical signal is received, adjusted by (e.g., reduced by) a feedback value, and amplified to provide an output signal. Noise is removed from the input signal based on the output signal. The feedback value is determined by integrating the output signal.
The present invention thus provides a fast, reliable and precise technique for filtering noise while avoiding false signal spikes.